The invention relates to a safety circuit arrangement for an electric drive unit, wherein the electric drive unit has a traction battery, an intermediate circuit capacitor connected in parallel with the traction battery, and an electric machine that can be supplied with electric power by the traction battery. The electric machine has a plurality of phases that can be connected to the traction battery via a controllable inverter having a plurality of switch elements.
Electric drive units designed in this manner can be used in a wide variety of ways. Among others, they can be used as a drive unit in a vehicle, wherein the vehicle may be in the form of a hybrid vehicle or in the form of an electric vehicle. In the case of a hybrid vehicle, not only the electric machine but also a further unit is used for the drive, normally an internal combustion engine, whereas an electric vehicle is driven exclusively by an electric machine. The electric machines used are normally designed as internal rotor machines, in which a rotatably mounted rotor is surrounded by a fixed stator. The stator produces a rotating magnetic field that takes along the rotor. The rotor has a rotor shaft that is operatively connected to a drive shaft of the vehicle. The electric machines used may be synchronous machines, particularly hybrid synchronous machines, which are preferably embodied as permanently excited synchronous machines. A hybrid synchronous machine is a permanently excited synchronous machine that additionally has a strongly pronounced reluctance effect, caused by a correspondingly chosen rotor geometry, that is jointly used for producing the torque acting on the rotor. A traction battery is a high-voltage storage, which can have a voltage level that may be in the order of magnitude of fully 250 to 450 volts. Preferably, a traction battery is constructed from Li ion battery cells.
To ensure safety links with an electric drive unit, particularly when the electric drive unit is arranged in a vehicle and the electric machine is in the form of a permanently excited synchronous machine, various safety measures need to be provided.
Firstly, care must be taken to ensure that the high voltage applied to the intermediate circuit capacitance can be reduced by discharging the intermediate circuit capacitance when particular conditions are present or arise, such as particular operating states of the vehicle. This safety measure is intended to ensure that, for example in the idle state of the vehicle, the intermediate circuit capacitance is not live and hence, for example when maintenance work is performed, there is no possibility of contact with the high voltage, which is potentially hazardous to human beings. A vehicle contains a safety circuit arrangement that is designed specifically for performing this safety measure.
Secondly, safety measures also need to be provided for the electric machine, particularly when it is a permanently excited electric machine. In the case of permanently excited electric machines, their design means that, during operation, the relative movement that occurs between the stator windings and the permanent magnets prompts a negative field voltage to be induced in the stator windings, which is referred to as the rotor voltage. As rotation speed increases, the rotor voltage rises until, during operation of the electric machine, it is in the region of the supply voltage provided by the traction battery for the electric machine. If there is now no voltage source connected to the intermediate circuit capacitance and hence to the intermediate circuit, i.e. the connection between the traction battery and the intermediate circuit capacitance or intermediate circuit is interrupted for any reason, then the rotor voltage can be present across the intermediate circuit capacitance. This can lead to damage to the intermediate circuit capacitance and/or components of the inverter, particularly to the switch elements. In order to avoid such damage, when particular conditions are present or arise, such as particular operating states, provision is made, specifically when the traction battery is not connected to the intermediate circuit capacitance, in particular, for the phases of a permanently excited electric machine to be shorted by appropriate actuation of the switch elements of the inverter. Also, for the purpose of performing this safety measure, a safety circuit arrangement designed specifically for this purpose is provided in a vehicle.
Both for the purpose of discharging the intermediate circuit capacitance and for the purpose of shorting the phases of the electric machine, particularly a permanently excited electric machine, the safety circuit arrangements designed for this purpose need to be actuated or to have an appropriate supply voltage applied to them. Particularly as far as this aspect is concerned, the known safety circuit arrangements are not yet optimum. Thus, in particular operating states of the vehicle, for example in the case of an accident in which, owing to external effects on the vehicle, it is no longer possible for the safety circuit arrangements to be supplied with electric power either by the traction battery or via the starter battery, a sufficient or temporarily continuous supply to the safety circuit arrangements is not guaranteed, which is why firstly discharge of the intermediate circuit capacitance to a level at which a voltage that is nonhazardous to human beings is established on the intermediate circuit capacitance and secondly persistent shorting of the phases of the permanently excited electric machine is not guaranteed.
Furthermore, the known safety circuit arrangements have a need for improvement also in respect of the number of components that are needed for implementing the respective safety measure, and as a consequence thereof in respect of the installation space needed for the design of the safety circuit arrangement, and also in respect of the heat generation that arises for the safety circuit arrangements.
It is therefore an object of the present invention to provide an improved safety circuit arrangement that, even when electric power can be supplied neither via the traction battery nor via the starter battery, allows both discharge of the intermediate circuit capacitance to a level at which a voltage that is nonhazardous to human beings is established and persistent shorting of the phases of an electric machine, particularly a permanently excited electric machine. In addition, the safety circuit arrangement is intended to be improved or optimized in respect of the number of electrical or electronic components required, in respect of the installation space required and in respect of the heat generation that arises during the operation thereof. That is to say that the safety circuit arrangement is intended to be constructed from as few components as possible, to require a very small installation space and additionally to have very low heat generation during operation. In addition, the production costs of the safety circuit arrangement are intended to be low.
This and other objects are achieved by a safety circuit arrangement of the type cited at the outset that has the following features: a discharging circuit that is designed, in its activated operating state, to take a prescribable discharge current from the intermediate circuit capacitance, a short circuit control circuit that is designed, in its activated operating state, to short at least some of the phases of the electric machine by actuating some of the switch elements, a supply voltage circuit that is designed to provide supply voltage on the basis of an input voltage that is supplied to it, wherein the input voltage supplied is an intermediate circuit voltage that is applied to the intermediate circuit capacitance, and an activation element that is designed to close an activation path when a switch-on condition is present, in order to activate the discharging circuit and the short circuit control circuit by providing the supply voltage.
The safety circuit arrangement according to the invention is based on a plurality of concepts. According to a first concept, the safety circuit arrangement of the invention has provision for a supply circuit arrangement to which, as an input voltage, the intermediate circuit voltage that is present across the intermediate circuit capacitance is supplied in order to be able to provide a supply voltage on the basis of this intermediate circuit voltage. Hence, a supply of electric power to the safety circuit arrangement is guaranteed even when supply is possible neither by the traction battery nor by the starter battery (voltage level 12 volts, for example), for example when the supply lines are interrupted. Hence, even if the traction battery and the starter battery fail simultaneously, it is possible both for the intermediate circuit capacitance to be discharged to a level at which a voltage that is nonhazardous to human beings is established and for the phases of a permanently excited electric machine to be persistently shorted.
According to a second concept, the safety circuit arrangement contains both a discharging circuit and a short circuit control circuit. Hence, synergies that arise can be used, which, by way of example, leads to a reduction in the components that are needed for implementing the discharging functionality, on the one hand, and for implementing the shorting functionality, on the other hand. Equally, the integration of both functionalities into a circuit arrangement reduces the installation space that is needed for the design of the circuit arrangement, i.e. the size of the board or printed circuit board that is needed for accommodating the circuit arrangement decreases. The printed circuit board and hence the safety circuit arrangement can be produced much more compactly. The reduction in the number of components, these being high-voltage resistors, power transistors and diodes, inter alia, additionally has the positive side effect that the heat generation on the board carrying the circuit arrangement and hence the introduction of heat into the board are lower. Furthermore, the smaller number of components and the lower installation space requirement mean an overall reduction in production costs.
According to a third concept, in combination with the safety circuit arrangement according to the invention, i.e. simultaneously, it is possible firstly for the phases of an electric machine to be shorted and secondly for an intermediate circuit capacitance and hence the intermediate circuit to be discharged. It is therefore possible to achieve an extremely high degree of safety for an electric drive unit with little complexity and with a high level of reliability. It is therefore simultaneously possible to discharge a DC voltage intermediate circuit and short all the terminals of an electric polyphase machine. This is achieved particularly by virtue of the activation element operatively connecting the supply voltage circuit, on the one hand, and the discharging circuit and also the short circuit control circuit, on the other hand, to one another via an activation path.
Before further advantageous embodiments of the safety circuit arrangement according to the invention are discussed, it should be mentioned at this juncture that the above reference to a permanently excited electric machine is not meant to have any restricting effect. It goes without saying that the safety circuit arrangement according to the invention can also be used for electric machines of a different design.
Advantageously, the discharging circuit is in a controllable form, so that the discharge current taken from the intermediate circuit capacitance can be established.
Preferably, the short circuit control circuit is designed, in its activated operating state, to short all the phases of the electric machine as a result of actuation of the switch elements. This measure achieves an extremely high degree of safety.
As already explained, the electric drive unit is preferably arranged in a vehicle. Accordingly, the switch-on condition advantageously exists when at least one of the following situations is present:                transfer of the vehicle from a driving operating state to an idle state,        shutdown of the traction battery, or        presence of a critical driving operating state that is different than the normal driving operating state.        
When the vehicle is transferred from a driving operating state, in which the vehicle is driven by the electric machine, to an idle state, in which the vehicle is stationary, and in which particularly the electric machine is no longer connected to the traction battery and hence to the intermediate circuit capacitance, care must be taken to ensure that the intermediate circuit capacitance is discharged. At the beginning of the idle state, the voltage that is present across the intermediate circuit capacitance corresponds to the high voltage provided by the traction battery. Accordingly, the onset of the idle state requires the intermediate circuit capacitance to be discharged, as far as possible very quickly, to a level at which a voltage that is nonhazardous to human beings is established. This is achieved with the safety circuit arrangement according to the invention. Alternatively, the switch-on condition can exist when the presence of a shutdown state is identified.
When shutdown of the traction battery is detected, care must be taken to ensure that firstly the intermediate circuit capacitance is discharged in order to reduce the voltage that is present across it, which corresponds to the high voltage provided by the traction battery directly at the beginning of shutdown of the traction battery, and that secondly the phases of the electric machine are shorted, so that no voltages can build up thereon. This is also achieved with the safety circuit arrangement according to the invention. Alternatively or additionally, a traction battery state condition can be sensed and evaluated, which provides evidence of the state of charge of the traction battery and/or of a fault that is present in the traction battery, for example.
If it is detected that a critical driving operating state that is different than the normal driving operating state is present, then safety measures likewise need to be taken. By way of example, a critical driving operating state may be a driving operating state in which very large accelerations, particularly lateral accelerations but also longitudinal accelerations, and/or high yaw angle speeds arise, all of which are an indication of a skidding process, as may exist in the case of an accident, for example, or in the case of a journey in which a driver demand means that the physically dependent limit values, for example in relation to static friction, particularly in relation to cornering, are exceeded. In this case, it is likewise advisable firstly to discharge the intermediate circuit capacitance as quickly as possible and secondly to short the phases of the electric machine. The aforementioned variables can advantageously each be sensed by use of suitable sensors.
Alternatively and/or additionally, a machine state condition can be sensed and evaluated.
In one preferred embodiment of the invention, the supply voltage circuit is a series circuit consisting of an electrical supply resistor and a zener diode. This is a simple, reliable and inexpensive measure to provide the supply voltage that is required for operating the safety circuit arrangement. The complexity or the requirement for implementing a supply voltage circuit is/are therefore reduced to a minimum degree. At the same time, the circuit constructed in this manner allows passive discharge of the intermediate circuit capacitance even when the discharging circuit is not activated.
In a further preferred embodiment of the invention, the discharging circuit is a series circuit consisting of an electrical load resistor and a controllable semiconductor element. This measure is also distinguished by its simple and inexpensive design, given simultaneously high reliability. At the same time, the discharge current can be established in a simple manner. Preferably, the semiconductor element is a bipolar transistor.
In a further preferred embodiment of the invention, the safety circuit arrangement additionally has a monitoring circuit. This monitoring circuit is designed to process an electrical signal that represents a temperature that is present on the load resistor. Hence, if a profile of the temperature of the load resistor is unfavorable, this temperature essentially being determined by the discharge current flowing through the load resistor, it is possible for temperature-influencing measures to be taken, particularly when the temperature approaches or has even already exceeded a prescribed limit value; in particular, it is possible for measures to be taken that result in the temperature no longer rising further, but preferably being lowered. Consequently, the monitoring circuit is designed to actuate the controllable semiconductor element on the basis of an ascertained monitoring result. As a result, it is possible to set, preferably to reduce, the value of the discharge current taken from the intermediate circuit capacitance. In the extreme case, the semiconductor element can be actuated such that a discharge line through which the discharge current flows is interrupted or opened, and hence no further discharge current flows, i.e. the discharge current assumes the value zero. The load resistor is the main load for the actively occasioned discharge of the intermediate circuit capacitance.
In one preferred embodiment of the aforementioned measure, the monitoring circuit contains a measuring bridge circuit constructed from electrical measuring resistors, wherein one of the measuring resistors is in the form of a temperature-dependent resistor. This is a particularly simply implemented measure for sensing the temperature that is present on the load resistor, that is additionally very reliable and simultaneously allows precise temperature sensing. Consequently, the temperature-dependent resistor is arranged physically close to the load resistor, so that the electrical signal to be processed by the monitoring circuit represents the temperature that is present on the load resistor as precisely as possible.
In one advantageous embodiment of the aforementioned measure, the temperature-dependent resistor has a negative temperature coefficient. Using a measuring resistor in such a form, it is a simple matter to implement reliable and precise sensing of the temperature that is present on the load resistor.
In one preferred embodiment of the invention, the safety circuit arrangement additionally has a stabilization circuit that has its circuitry arranged between the supply voltage circuit and the activation element. This measure ensures that a sufficiently large current can be provided for the actuation of the switch elements of the inverter that is required for shorting the phases of the electric machine. This ensures reliable shorting of the phases of the electric machine.
Advantageously, the inverter has a multiplicity of half-bridges, wherein each half-bridge has a first and a second switch element. The first switch element is connected to a supply terminal of the traction battery and the second switch element is connected to a ground terminal of the traction battery. In the safety circuit arrangement according to the invention, the short circuit control circuit is preferably designed to actuate the second switch elements connected to the ground terminal. This is a measure that allows actuation of the switch elements for shorting the phases of the electric machine without great circuit complexity. Alternatively, however, it is likewise contemplated for the short circuit control circuit to be embodied such that the switch elements connected to the supply terminal of the traction battery are actuated for the purpose of shorting phases of the electric machine.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.